This invention relates to the field of rubber modified asphalts for use in sealing and paving materials.
It has long been recognized that the addition of particulate rubber to hot asphalt, where the rubber is blended and reacts fully or partially with the oils in the hot asphalt, provides a paving wear surface that has improved properties over that of asphalt alone as a paving material. In particular, rubber modified asphalts appear to have much longer effective lifetimes under a wide environmental condition, are less prone to the early development of stress cracking, and are more resistant to at least medium load traffic wear than plain asphalts. In addition, the rubber appears to have beneficial effects in reducing the evaporation of volatile oils from the asphalt, which is the principal predecessor to asphalt aging and failure, especially under the influence of heavy traffic, hot, dry climates, and excessive sunlight.
It is now generally recognized that the initial success in using crumb sized rubber particles to mix with asphalt is in part due to the nature of the rubber used. Asphalts have long been recognized as having extremely varying properties depending upon the source petroleum from which they are derive, and these properties are largely dependent upon the nature of the complex heavy oil within the asphalt. Asphalt may in fact be blended and cut with various oils to improve their properties. A detailed description of the known prior art rubber asphalt mixtures and solution products is given in Huff, U.S. Pat. No. 4,166,049, incorporated by reference herein, which points out that the nature of the rubber type and the asphalt oil content has been considered material in the prior art coarse crumb rubber.
Crumb rubber or particulate rubber is the product of reclamation processes, and has as a source material scrap rubber products. As a result, it is not a well defined material, and may consist of varying proportions of both natural rubbers and synthetic rubbers. Such rubbers are described more fully in Werner Hoffman, Rubber Technology Handbook, Oxford University Press (1989) (English Trans. of Kautschuk-Technologie, 1980). Various natural and synthetic rubber compositions vary considerably in their solubility in various oils and solvents, and some are far more resistant to devulcanization, which must occur if the rubber is to react fully with asphalt.
In the prior art, crumb rubber has generally only been widely available in sizes greater than a minus 30 mesh, due to limitations of rubber reclaiming technology. Only two processes are known for producing finer rubber; one is a cryogenic process which has only been used in test quantities due to its expense; the other is mill grinding of a crumb rubber slurry. The product of the latter process is referred to herein as ground particulate rubber.
The form of the rubber used with the asphalt affects considerably the properties of the resultant mix. Early work in rubber asphalts used primarily natural rubbers which reacted readily with the asphalts forming a suitably homogeneous blend of desirable properties. Early attempts to use synthetic rubbers with asphalts were not so successful as the synthetic rubbers generally resist the devulcanization and are more resistant to entering the gel state which is a typical intermediate encountered in creating a rubber solution with asphalt as part of the rubber asphalt reaction. This gel state is shown in McDonald, U.S. Pat. Nos. 3,891,585 and 4,069,182. McDonald discloses gelled compositions result when using crumb rubbers larger in particle size than minus 8 to minus 30 mesh.
The size of crumb and particulate rubber is difficult of definition as the particles are not uniform in shape, and tend to aggregate or clump together. Since there is no defined dimension on such a particle for measurement, the usual definition is to sort such particles by standard screens, and to define size by the screen mesh through which substantially all particles pass. Thus, such a screen size, for example a minus 16 mesh, means that substantially all particles are smaller than the mesh size of minus 16 mesh.
Thus, more recent work involving synthetic rubber crumb and asphalts shows that the resulting mixture, especially at the higher percentages of rubber, such as 20% and above, tends to form an extremely viscous gel which is resistant to spreading and creates great handling problem. Further, the mixture continues to react with time and thus must be mixed within a few hours prior to application, often must be cut with various extender oils in order to provide a mixture of suitably reduced viscosity to permit ready spreading and cannot be held for any length of time at an elevated temperature before applying.
As a result, rubber asphalt mixtures must now be mixed essentially on site or immediately prior to application, must be mixed in relatively small batches, requiring that the mixing equipment be localized and thus produce considerable quality and consistency problems in their application.
The additional handling required for on site mixing of rubber and asphalt to produce rubber asphalt mixtures also increases the cost of the mixture and introduces a complicating factor into the otherwise well developed central asphalt mixing and distribution industry.